On The "Low" A-Factors of Alkyl Radical Isomerization

Abstract

Recent chemical activation studies of isomerizations of alkyl and alkenyl radicals involving 5, 6, and 7 member ring complexes are shown to be consistent with earlier thermal studies and support the conclusion that the A-factors of these reactions are "low", i.e. substantially less than 1011 s−1. A possible explanation, consistent with transition state theory, is given in terms of configurational entropy.

Cyclization and the conformation of hydrocarbon chains

“…Simulations of hydrocarbon chain cyclization in terms of the RIS model indicate that (^cf,cy/^cf,total) ~10'3 for 30 carbon chains. 80 This represents, from the first term on the right-hand side of eq 11 a contribution of -14 cal deg'1 mol'1 to the cyclization entropy. These simulations also indicate that for unstrained rings, the fraction of bonds in the gauche and trans rotational states is very similar to that in the acyclic molecule.52 '80'81 Consequently, (d In Zcffiy/dT) «= (3 In Zcf,total/dT), and eq 11 simplifies, for large-ring formation, to where W(O) is the cyclization probability of the chain.…”

Free-Radical Rearrangements

“…Because the transition state for the isomerization is conformationally very restrictive, the preexponential factor is "low" (63,601,602) in compari son with the "normal" value of CÖ. IO 13 sec -1 found for many other types of unimolecular reaction.…”

Computer model for flexible hydrocarbon chain behavior in the mass spectrometer

“…The number of reactive conformations is a direct estimate of the configurational entropy for reaction. 23 The ratio (reactive conformations)/(all conformations) is related to the configurational component for the entropy of activation for hydrogen abstraction. The term v contains activation entropy in eq 8.…”